1. Field of the Invention
The present invention relates to a transparent substrate having a rain sensor for detecting adhesion or presence of water drops upon the transparent substrate, such as a windshield, by using a light emitting element and a light receiving element.
2. Description of the Related Art
Conventionally, a device for automatically actuating a wiper by detecting the adhesion of rain drops upon a windshield of an automobile is known, and as such a substrate provided with such a detecting function is already known as shown in FIG. 5.
In Japanese Laid-open Patent No. Sho 60-216245 (1985), there is disclosed a device wherein the detection sensitivity is poor in the case where light from a light source is propagated within a glass plate by total internal reflection therein and is incident upon a light receiving element through a prism, as in the detection method mentioned above. Therefore, the angle of the prism is set up so that light from a water drop is incident upon the light receiving element only when water drops adhere to or are present upon the glass plate.
Further, in Japanese Laid-open Patent No. Sho 62-163949 (1987), there is disclosed a construction in which two light sources are prepared, wherein the incident angle of light flux from one of the light sources is set at a value greater than the critical angle for total internal reflection to occur upon a detection surface, while light flux from the other is less than that of the critical angle thereupon, thereby enabling discrimination as to whether water (drops) are adhering to or are present.
Moreover, in Japanese Laid-open Patent No. Hei 8-261974 (1996), there is disclosed a construction in which comb-shaped transparent electrodes are provided inside a glass plate, wherein change in electrostatic capacitance between these transparent electrodes, which is caused by the adhesion or presence of the water drops, is detected so as to open or close the window and/or to control a heater.
Moreover, in Japanese Laid-open Patent No. Hei 6-509652 (1994) (prosecuted via PCT application), there is disclosed a construction in which a sensing unit made of, for example, a prism is adhered, by means of an intermittent layer having two (2) adhesive surfaces, onto an interior surface of a windshield, consequently sensing the water drops upon the outside surface of the windshield and controlling the operation of the wiper for the windshield.
In the prior art comprising the optical sensing function as mentioned above, the prism(s) is/are essential parts for introducing the light into the glass to undergo total internal reflection, and the prism(s) must be closely contacted upon the glass surface. Therefore, it is difficult and costly to install such devices. In particular, since the glass in most windshields is formed into a curved design, it is difficult to achieve closely contacted adhesion. This is a problem.
Further, it is necessary that the prism and the glass surface must match each other in the refractive index thereof for prohibiting unnecessary reflection at the boundary surface between them (i.e. refractive index matching). For this purpose, a layer for the matching can be provided between them, however, this increases the number of processing steps during manufacturing and is, therefore, costly. This is also a problem.
On the other hand, a system that detects the change in electrostatic electric capacitance, the electric resistance between the electrodes, or the like is inferior in durability and sensitivity to that of the optical type. In particular, as the front windshield glass for use in an automobile, it is important that the sensed signal correspond to or vary with the visibility seen by a driver or a fellow passenger, and therefore, an optical sensor is preferable.